The detection of clinically relevant substances in biological samples, such as cells and pathogens, is important in the field of diagnostics. Many traditional detection strategies are based on optical techniques. However, such techniques are often affected by deleterious effects such as light scattering, absorption and autofluorescence. Minimizing such effects can require extensive sample purification prior to the recording measurements.
Detection strategies employing magnetic nanoparticles offer unique advantages over traditional detection methods. For example, biological samples exhibit negligible magnetic background, and so the use of magnetic nanoparticles provides the opportunity to obtain very sensitive measurements in samples without subjecting the samples to significant pre-processing steps.
A magnetic nanoparticle may be comprised of an inorganic magnetic core and a biocompatible surface coating that stabilizes the particle in physiological conditions. By applying suitable surface chemistry, functional ligands can be incorporated onto the nanoparticle in order to confer molecular specificity.
When magnetic nanoparticles are placed in an external field, each particle creates a local magnetic field, which increases the field inhomogeneity. This has the effect of perturbing the coherent precessions of water proton spins when water molecules diffuse in the proximity of the nanoparticles. As a consequence, the net effect is a change in the magnetic resonance signal, which is measured as a shortening of the longitudinal (T1, spin-lattice) and transverse (T2, spin-spin) relaxation times.
The term “T2” refers to the spin-spin relaxation time constant characterizing the signal decay, and can be represented by the equationMxy(t)=Mxy(0)e−t/T2 where Mxy is the transverse component of the magnetization vector tipped down by a radiofrequency (RF) pulse.
Magnetic nanoparticles have previously been used to detect biological targets such as bacteria and mammalian cells. Target-specific magnetic nanoparticles are used which tag cell-surface biomarkers, thereby imparting a magnetic moment. The increase in the relaxation rate R2=1/T2 is directly proportional to the number of nanoparticles bound to a target (and also indicative of the amount of marker surface markers). The change of R2 and hence T2 can therefore be used to detect the presence of cells in a sample solution (Shao et al., Beilstein J. Nanonotechnol; (2010); 1; pages 142-154). However, the methods described in the prior art require removal of unbound magnetic nanoparticles to ensure sensitivity of the assay, which is typically achieved by filtration. Where detection has been achieved by employing a micro-MR device, the device requires a filtration membrane at the end of the reaction flow pathways for unbound magnetic nanoparticles to be filtered out (Lee et al., Angew Chem Int Ed Engl; (2009); 48; pages 5657-5660).
The need to remove unbound magnetic nanoparticles from the bound magnetic nanoparticles gives rise to a number of problems. For example, employing a microfluidics network and filtration membrane add to the cost of MR detection devices. The additional filtration step can also increase the cost of sample preparation, a factor which is particular significant in developing countries where affordable testing techniques are paramount. Furthermore, the need for an additional separation step exposes the technician performing the diagnostic test to potentially hazardous substances.